1. Field of the Invention
The present invention relates to a source-coupled FET logic (SCFL) circuit having GaAs-FETs and, more particularly to an SCFL circuit for use in a high-speed digital integrated circuit.
2. Description of the Related Art
An SCFL circuit is known in the art as one of the basic GaAs IC circuits for use in high-speed digital integrated circuits. The SCFL circuit has the structure illustrated in FIG. 1.
As is shown in FIG. 1, the SCFL circuit comprises a switching section 10 and a source-follower section 20. The source-follower section 20 is connected to receive an output from the switching section 10.
The switching section 10 has two FETs 11 and 12 having their sources coupled together, thus forming a differential pair. Complementary signals IN and IN are input to the gates of the FETs 11 and 12, respectively, and two signals of the opposite polarities are obtained at the drains of the FETs 11 and 12. The section 10 further comprises a constant current source 13 and three resistors 14, 15 and 16. The resistors 14 and 15 are loads to the FETs 11 and 12, respectively. The resistor 16 functions as a level-shifting element.
The source-follower section 20 is designed to supply a signal which is at a level suitable for the next-stage circuit (not shown), thereby to drive the next-stage circuit. The source-follower section 20 comprises two FETs 21 and 22, a first group of diodes 231 to 23n, a second group of diodes 24.sub.1 to 24.sub.n, and two constant current sources 25 and 26. The FETs 21 and 22 are connected to receive the signals supplied from the drains of the FETs 11 and 12, respectively. The diodes 23.sub.1 to 23.sub.n cooperate to shift the level of the signal output by the FET 21. The diodes 24.sub.1 to 24.sub.n cooperate to shift the level of the signal output by the FET 22. The constant current sources 25 and 26 are connected to the diodes 23.sub.n and 24.sub.n, respectively.
The source-follower section 20 generates two signals Q and Q from the input signals IN and IN. The signals Q and Q are supplied to the next-stage circuit.
In the conventional SCFL circuit described above, the diodes 23.sub.1 to 23.sub.n and the diodes 24.sub.1 to 24.sub.n, which are used as level-shifting elements in the source-follower section 20, have a resistance component each. Hereinafter, this resistance component will be regarded as an AC component and, thus, will be referred to as "impedance." Due to the impedance, the SCFL circuit has but an insufficient ability of driving the next-stage circuit. Consequently, the conventional SCFL circuit cannot serve to constitute a high-speed digital integrated circuit.
In order to increase that ability of driving the next-stage circuit, it is necessary to use a large-current source and to supply a great current to the source-follower section 20. Obviously, the use of a large-current source results in a great power consumption.